The present invention relates generally to switched resistors that are used to emulate a resistance in an integrated circuit, and more particularly, to a pseudo-random switched resistor that produces a very high resistance.
Switched resistors have been conventionally used to provide high input impedance values for amplifiers used in integrated circuits. The resistor is a plate of conductive material that also has a capacitance associated with it. Therefore, resistance and capacitance are linearly related. What is desired is to have a high value resistor with a low capacitance value. This has not been achieved in the past. Conventional large value resistors have large capacitance values which pass less signal to the amplifier. Furthermore large resistors typically take up a large amount of chip area and this is not desirable.
Therefore, what has conventionally been done is to use a switched resistor, which is a low value (2 megohm) resistor that is switched into and out of the circuit. The resistor is switched at a low duty rate (1/512, for example), and the effective resistance of the switched resistor is the actual resistance value divided by the duty rate, which produces a very large effective resistance value (the resister value times 512, for example). If the clock frequency used to switch the resistor is close to the frequency of the signal that is sampled, aliasing occurs. In a worst case situation, a DC component of the input signal is generated which results in a DC offset to the amplifier.
Furthermore, the use high clock frequencies (on the order of a gigahertz) is impractical. In addition, charge injection occurs in the switched resistor, and at the higher clock rates so much charge injection occurs that the high resistance effect is lost.
In view of the above, conventional switched resistors have used a fixed switching frequency. Switched resistors using fixed frequency switching require the input signal frequency to be much less than the switch frequency to avoid modulation. This imposes a limit on the input signal bandwidth or the maximum value of the equivalent resistance. If the input signal frequency happens to be in the neighborhood of or greater than the switch frequency, modulation occurs. The magnitude of the modulation may be as big as the amplitude of the input. This means the output signal has an amplitude error of as much as 100%.
The presence of modulation thus imposes a limit on the switch frequency and input signal frequency. Ultimately, this sets a limit on the attainable equivalent resistance. The equivalent resistance is inversely proportional to the duty cycle of the switch. To obtain the highest equivalent resistance, it is desirable to make the switch on-time as short as possible and the switch frequency as low as possible. The on-time is limited by the offset voltage caused by charge injection of the switch. The period is limited by the input frequency to avoid modulation. Thus high resistance and high input signal frequency are incompatible.
In view of the above, it is an objective of the present invention to provide for a switched resistor that emulates a high resistance and that overcomes the problems outlined above.